[BUG] Fix admin layout

- Fix the layout of admin pages, it previously was full-width and had
the alert at the incorrect place and within an container.
- Make the placement of the alert consistent with other pages, inside
`flex-container-main` and not wrapped around a container.
- We have to revert 145bebc829, as this
expected that the page contain provided padding, this was provided by
the incorrect placement of the alert. As well isn't consistent with how
other pages are being shown, non-full width. The solution to the
described problem isn't optimal and should rather be fixed with the tables.
- Reverts 145bebc829
- Resolves #3082

(cherry picked from commit c2d0f64340)

templates/admin/layout_head.tmpl

@@ -1,11 +1,9 @@
 {{template "base/head" .ctxData}}
 <div role="main" aria-label="{{.ctxData.Title}}" class="page-content {{.pageClass}}">
-	<div class="ui container">
-		{{template "base/alert" .ctxData}}
-	</div>
-	<div class="ui container fluid padded flex-container">
+	<div class="ui container flex-container">
 		{{template "admin/navbar" .ctxData}}
 		<div class="flex-container-main">
+			{{template "base/alert" .ctxData}}
 			{{/* block: admin-setting-content */}}
 
 {{if false}}{{/* to make html structure "likely" complete to prevent IDE warnings */}}